Generally, a capacitive microphone utilizes a capacitance between a pair of electrodes which are facing each other to generate an acoustic signal. A MEMS microphone may be manufactured by a semiconductor MEMS process to have an ultra-small size.
The MEMS microphone may include a bendable diaphragm and a back plate which is facing the diaphragm. The diaphragm can be a membrane structure to generate a displacement due to the acoustic pressure. In particular, when the acoustic pressure is applied to the diaphragm, the diaphragm may be bent upwardly or downwardly due to the acoustic pressure. The displacement of the diaphragm can be sensed through a change of capacitance between the diaphragm and the back plate. As a result, an acoustic wave can be converted into an electrical signal for output.
The characteristics of the MEMS microphone can be determined by measuring various factors such as a frequency response, a pull-in voltage, a total harmonic distortion (THD), sensitivity, etc.
The frequency response may indicate how the sensitivity of the MEMS microphone varies with respect to the frequency. The less sensitivity changes with frequency, the better performance of the MEMS microphone has.
In particular, the performance of a MEMS microphone can be safely secured when the cut-off frequency should be less than about 3 dB. Here, the cut-off frequency of the MEMS microphone corresponds to a difference between the sensitivity at about 1 kHz and the sensitivity at about 100 Hz.
Furthermore, the cut-off frequency characteristic may be improved, as an inlet area where the acoustic pressure moves into becomes large or a pressure where the acoustic pressure moves out from the MEMS microphone becomes higher. However, it may be difficult to enlarge the inlet area for the acoustic pressure because the enlarged outlet area may cause the diaphragm or the back plate to sag down, and the minimum area occupied by other components may be further required.
On the other hand, a total harmonic distortion (THD) may indicate a phenomenon that unnecessary harmonic components are generated in the audio signal and a distortion occurs. The distortion components correspond to component tone originally not included in audio signal input, which may cause reproduction sound quality to be degraded. Therefore, the MEMS microphone may be required to be configured so that the THD does not exceed the proper level. Further, when the flexibility of the diaphragm is low, the THD may become relatively high to deteriorate the quality of the MEMS microphone.